Over the years, a variety of power tools, such as rotary sanders, have been developed for use in finishing the surface of a workpiece. Generally speaking, these tools rotate an abrasive material across the workpiece to scour or sand away the surface of the workpiece.
A rotary sander has a motor which drives a substantially planar disc, with abrasive elements bonded to it, in a circular motion across the work surface. The abrasive particles scrape off the top coating or surface of the work piece. A common type of disc used with these sanders comprises a fiber substrate onto which sharp, abrasive particles are securely bonded with a phenolic resin coating. The cured resin coating locks the sharp abrasive particles onto a fiber substrate. Rotary sanders are relatively inexpensive and easy to use and, accordingly, are very popular in surface finishing.
Two factors that must be considered when designing an abrasive disc for a rotary disc system are the life of the disc and the drive system that will rotate the disc. If the life of the disc is relatively short, the operator will need to change the disc quite often during a finishing operation. In such cases, it is desirable to design the disc to be quickly and easily removable from the drive unit. This is especially critical for small diameter discs which wear more quickly than larger diameter discs. Small diameter discs are finishing discs which are typically mounted to conventional 1/4 inch or 6 mm drive arbors.
The drive system chosen for attaching small diameter discs to power tools varies depending on the type of finishing disc. For instance, finishing discs which include a thick plastic backing plate usually have a metal insert press-fit or bonded into a hole formed through the backing plate. The metal insert has standard Unified coarse internal threads (UNC) formed in it which mate with corresponding UNC external threads formed on the arbor. It has been common practice to utilize a number of threads per inch of between 18 and 24. However, the large number of threads per inch also requires that the disc be turned a considerable number of times before it is fully engaged with the arbor. This increases the time it takes to change a worn disc and, thus, increases the time it takes to complete a surface finishing operation.
Small diameter discs that do not have a separate rigid backing plate but, instead consist of a thin layer of resin impregnated fiber material, utilize a different mounting arrangement. For these discs, a metallic grommet is adhesively attached to the back of the resin fiber material. Since the thin layer of resin impregnated fiber material does not have much stiffness, a separate universal support, called a blending disc holder, is used to add rigidity to the disc during use. The holder is made from rubber material and is generally conical or bell shaped. The wide end of the holder is flat and has a male threaded mount imbedded in the center. The threaded mount is configured to engage the grommet to attach the disc to the holder. The threads on the male mount are typically not standard UNC threads but, instead, have about eight threads per inch. The narrow end of the holder has a metal insert imbedded into it which has a set of female (internal) UNC threads formed in the insert. The female UNC threads are 1/4-20-UNC. The female threads engage with complimentary male (external) threads formed on a separate drive arbor. The advantage to this type of arrangement is the ability to change discs without removing the drive arbor from the power tool. One deficiency with this arrangement is that the grommets do not provide adequate support. It is quite common for the grommets to tear off the backing plate or disengage from the threads. Also, the combination of the holder and the disc is relatively heavy, increasing operator fatigue over long periods of use. The holder and disc combination is also relatively expensive.
Some small diameter discs are designed with arbors fixedly attached to the disc. The arbors are engaged with the drive system through a conventional chuck arrangement. A chuck key is used to tighten the jaws of the chuck around the arbor. When it is time to replace the disc, the operator must locate the chuck key and disengage the chuck jaws from the arbor. The disc/arbor combination is then discarded and replaced. This type of mounting arrangement is costly and is time consuming when replacing discs.
Another concern that must be addressed occurs when designing a drive system for attaching a finishing disc to a drive tool with a brake. The braking system on these types of tools provides relatively instantaneous stopping of the drive arbor. If the drive system is not designed correctly, the angular momentum of the finishing disc could result in disengagement of the disc from the drive arbor, with the danger of a disc flying off and striking someone.
A need therefore exists for an improved drive system for a finishing disc which permits easy engagement and disengagement from a power tool while prohibiting tear-out of an arbor from a disc. A need also exists for a drive system that prevents disengagement of the finishing disc when used on a drive tool with a brake.